# Lens Combination Question.

## Homework Statement

Two 28.0-cm-focal-length converging lenses are placed 16.5 cm apart. An object is placed 36.0 cm in front of one lens. Where will the final image be formed by the second lens?

## Homework Equations

1/do + 1/di = 1/f

## The Attempt at a Solution

for the first lens:
1/do + 1/di = 1/f
1/di =1/28 - 1/36 = 8/1000
di = 125cm

This is where I get confused I used 125cm as my do2 but the solution tells me I have to do 16.5cm - 125cm. My question is since the image due to lens 1 is 125cm away from lens 1, why would i have to subtract the lens separation in order to get do2? I know that do2 is a negative number but not why i have to take into account the lens seperation.

Thanks for you help

ehild
Homework Helper
Any distance is defined from something. Distance from a wall, distance from a town and so on. How is the object distance do defined?

ehild

The object distance is defined as the distance between the object and the lens.

for the first lens i got an image distance of 125cm. This goes beyond the second lens so to consider the first image as the second object wouldn't i have to subtract the distance in between the two lens and the distance between the first object and the first lens. Im confused as why they only subtract the distance in between the two lens.

Thanks again for all your help!

ehild
Homework Helper
The object distance is defined as the distance between the object and the lens.

for the first lens i got an image distance of 125cm. This goes beyond the second lens so to consider the first image as the second object wouldn't i have to subtract the distance in between the two lens and the distance between the first object and the first lens. Im confused as why they only subtract the distance in between the two lens.

Thanks again for all your help!

Iurod, why didn't you draw the set-up of these two lenses?
All distances are meant from the lens. The first image is 125 cm from the first lens. The second lens is at D distance from the first lens. What is the distance between the second lens and the first image?
If it is difficult to understand it with lenses, just think that the playground and the school are at the same direction from your home. The distance of the playground is 400 m from your home. The distance of the school is 1000 m from your home. What is the distance between the playground and the school?

ehild

#### Attachments

• twolens.JPG
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